1. Field of the Invention
The present invention relates to a system for controlling coupling force of a fluid coupling and, more particularly, to a system and a method for controlling coupling force of a fluid coupling during deceleration, containing control of output of a combustion engine, a fluid coupling being interposed between the combustion engine and driving wheels, and a system and a method being so adapted as to suspend supply of fuel to the combustion engine, i.e. to cut fuel off.
2. Description of Related Art
It is known that a fluid coupling is controlled so as to enhance its coupling force when the vehicle is decelerated, in order to avoid a rapid fall of the number of engine revolution during deceleration. Japanese Patent Laid-open Publication (kokai) No. 99,763/1986 discloses technology of switching to feedback control after the coupling force of the fluid coupling is elevated to a predetermined value by feedforward control, when the depression of the accelerator pedal is released, i.e. when the accelerator pedal is released. More particularly, a torque converter equipped with a lockup clutch is interposed between the combustion engine and the driving wheels, and the system is so arranged as to control the lockup clutch in the coupling direction when the vehicle is decelerated. This system enables the number of engine revolution to be prevented from decreasing rapidly during deceleration because the speed at which the number of engine revolution is slowed down in accompany with the release of the accelerator pedal by the force inputted to the combustion engine from the driving wheels upon deceleration.
On the other hand, it is also known, for example, as disclosed in Japanese Patent Laid-open Publication (kokai) No. 126,439/1983 that the supply of fuel to the combustion engine is suspended, i.e. the control of cutting fuel off is performed. The control of cutting the fuel off is performed within a range in which the combustion engine rotates at the predetermined number of revolutions, i.e. in a fuel-cut region. In other words, when the number of engine revolution reaches the number of engine revolution for cutting fuel off due to the release of the accelerator pedal, the control of cutting the fuel off is started. On the other hand, the supply of the fuel to the combustion engine is then resumed, i.e. fail-recovered, when the number of engine revolution has reached the number of engine revolution for recovering and resuming the supply of the fuel.
When the control of elevating the coupling force of the fluid coupling is combined with the control of cutting the fuel off, the quantity of consumption of fuel can be reduced while a feel of engine brake can be improved during deceleration. In other words, it can be noted as a matter of course that such a feel can be improved when the fluid coupling interposed between the combustion engine and the driving wheels is brought into an appropriately coupled state, i.e. a state in which the lockup clutch slips to an appropriate extent, while lengthening the period of time during which the number of engine revolution exists in the fuel-cut region by the input from the side of the driving wheels, thereby permitting the period of time for controlling the cut-off of fuel to be lengthened.
Further, U.S. Pat. No. 4,957,194 discloses technology of controlling the state in which the fluid coupling slips by a combination of feedforward control with feedback control. More specifically, the feedforward control is performed during the period of time ranging from the time when the control of cutting fuel off has been started to the time when the number of engine revolution reaches a predetermined value, and the feedback control is performed thereafter. This technology adopts the number of engine revolution larger than the number of engine revolution for recovering or resuming the supply of fuel, as the predetermined number of engine revolution.
Furthermore, U.S. Pat. No. 4,953,679 discloses technology in which a boundary region is interposed between a region in which the supply of fuel is cut off and a region in which the fuel is supplied, and the state in which the fluid coupling slips becomes large in the boundary region.
It is noted herein that it is of significance to set the timing for starting the control of cutting fuel off in a combination of the control of the slipping state of the fluid coupling with the control of cutting fuel off during deceleration. In other words, if the control of cutting fuel off would be started when the timing for starting cutting the fuel off would be too early, i.e. if the coupling force of the fluid coupling would be too small or if the state in which the fluid coupling slips is too large, the number of engine revolution is caused to be reduced rapidly, thereby immediately resuming the supply of fuel or performing fail recovery, so that it is not appropriate from the point of view of improvements in fuel economy. On the other hand, if the control of cutting fuel off would be started when the timing for starting the control of cutting fuel off is too late, i.e. when the coupling force of the fluid coupling would be too larger or the slipping state of the lockup clutch would be small, the problem may occur that a shock is caused due to a variation in torque due to the cut-off of the fuel.